This invention relates to a device for measuring magnetic field, electric current and the like by using the Faraday effect (magnetooptical effect), and more particularly to a specific type thereof which detects Faraday rotation angle for measuring the same.
Heretofore, widely known is a device which measures electric current flowing through a high voltage, heavy current electric apparatus utilizing the fact that the polarization plane of polarized light passing through a magnetooptical substance (Faraday rotor) is rotated in response to magnetic field produced by the electric current flowing around the magnetooptical substance. Such a device is more advantageous over those utilizing current transformers, in that it can be easily insulated from the high voltage of the electric apparatus, and that it does not disturb magnetic field created in the apparatus. Furthermore, the frequency characteristic of the device is better than those utilizing the current transformers.
However, since the known measuring device detects the rotated angle of the polarization plane in the form of a light intensity, the measured results tend to be affected by the loss in the optical paths. Furthermore, the results of the measurement are not proportional to the rotating angle of the polarized plane, and in a case where the measured results are converted into electrical signals by using a photoelectric converter, such as a photodiode, the electric signals tend to be deviated by a drift in the characteristics of the converter caused by temperature variation or the like.
These difficulties of the conventional device utilizing the Faraday effect will be further described in detail with reference to FIG. 1.
In FIG. 1, a laser light source 1 delivers a laser light beam 8, which is an electromagnetic wave having a component electric field E.sub.1 oscillating in a plane (or a plane of polarization) at a frequency .omega.. The laser light thus represented by the component electric field E.sub.1 is delivered to a Faraday rotator 2.
An orthogonal coordinate system comprising x, y and z axes is assumed on the entrance side of the Faraday rotator 2 with the z axis extending along the center line of the rotator 2. Furthermore, orthogonal coordinates .xi. and .eta. are assumed on the output side of the Faraday rotator 2 to extend perpendicularly to the z axis extending along the center line of the rotator 2.
The component electric field E.sub.1 of the laser light beam 8 delivered from the laser light source 1, with the polarization plane thereof disposed at an angle of 45.degree. to the x axis, then transmits through the Faraday rotator 2 in the direction of the z axis, and is delivered to a Wollaston prism 4. The Wollaston prism 4 splits the linearly polarized light into two components 9 and 10.
An electric conductor 3 is wound around the Faraday rotator 2 as shown in FIG. 1. An electric current I flowing through the conductor 3 induces in the direction of z axis a magnetic field H.sub.z proportional to the current I. The magnetic field H.sub.z rotates the polarization plane of the laser light passing through the Faraday rotator 2 by an angle of F.degree. from its original position forming 45.degree. to the x axis. That is, the light output E.sub.2 delivered from the Faraday rotator 2 is a linearly polarized light having a polarization plane rotated by an angle of 45.degree.+F.degree. from the x axis.
The angle F rotated by the rotator 2 is defined by the following equation EQU F=V.sub.k .multidot.H.sub.z .multidot.L
wherein V.sub.k is the Verdet constant of the magnetooptical substance forming the Faraday rotator, and L represents the length of the Faraday rotator along the z axis. Utilizing this relation, the magnetic field H.sub.z and hence the current I proportional to the magnetic field H.sub.z can be measured by detecting the rotated angle F of the polarization plane.
Assuming that the amplitude of the component electric field E of the linearly polarized light delivered from the laser light source 1 is equal to a, the laser light having a single frequency .omega. is expressed by EQU E.sub.1 =a sin .omega.t
Since the polarization plane of the component electric field E.sub.1 is disposed at 45.degree. with respect to the x axis, the x axis and y axis components E.sub.x and E.sub.y of the electric field E.sub.1 can be expressed as follows: ##EQU1## When the laser light is passed through the Faraday rotator, the polarization plane thereof is rotated by an angle of F.degree. as described above, and therefore the components E.sub..xi. and E.sub.72 along the .xi. and .eta. axes of the light output E.sub.2 outputted from the Faraday rotator can be expressed as follows: ##EQU2##
Thus the light output E.sub.2 having the components E.sub..xi. and E.sub..eta. is also a linearly polarized light with a rotated angle .theta.=45.degree.+F. of the polarization plane calculated according to the following equation. ##EQU3## Furthermore, from equation (2) the amplitude .vertline.E.sub.2 .vertline. of the light output E.sub.2 can be calculated as ##EQU4## showing that the amplitude of the light output E.sub.2 is equal to the amplitude of the input laser light.
The Wollaston prism 4 splits the light output E.sub.2 into two components 9 and 10 which are .xi. and .eta. axes components E.sub..xi. and E.sub..eta., respectively. The component 9, that is the component E.sub..xi., is received by a photodiode 5, while the component 10, that is the component E.sub..eta., is received by another photodiode 6. The photodiodes 5 and 6 convert the components 9 and 10 into electric signals I.sub.86 and I.sub.72 which are proportional to the light intensities .vertline.E.sub..xi. .vertline..sup.2 and .vertline.E.sub.72 .vertline..sup.2 of the two components 9 and 10, respectively. An electronic circuit 7 calculates (I.sub..eta. -I.sub..xi.)/(I.sub..eta. +I.sub..xi.). Since ##EQU5##
In tne conventional device shown in FIG. 1, the detected value is expressed in terms of sin2F as shown in equation (3), whicn is not proportional to the Faraday rotation angle F. Accordingly, another circuit for calculating the following relatron is required. ##EQU6## Furthermore, since the same result is obtained for a variation of F of 180.degree., there is another difficulty that the measuring range of F is restricted to .+-.90.degree..
The most serious difficulty of the device shown in FIG. 1 lies in that the characteristics of the photodiodes 5 and 6 tend to drift when the diodes convert the light intensities .vertline.E.sub.86 .vertline..sup.2 and .vertline.E.sub..eta. .vertline..sup.2 into electric signals I.sub..xi. and I.sub..eta.. More specifically, a dark current tends to flow in each photodiode besides the electric signal I.sub..xi. or I.sub..eta.. The dark current varies in accordance with the temperature variation of the photodiode, thereby causing a drift in the operational characteristics of the photodiode. When the adverse effect of the dark current is desired to be eliminated, a complicated circuit must further be provided in addition to those indicated in FIG. 1.
Furthermore, when it is desired to measure accurately, the photoelectric conversion ratios of the two diodes 5 and 6 must be equal. However, when considering variation in characteristics of the photodiodes over many years it is extremely difficult to maintain the photoelectric convers:on ratios of the two diodes to be euqal.